A dowel of the forenoted type is for example used for fixing a component to a for example, plasterboard consisting of a core of friable material, plaster between two layers of board. The difficulty in fixing a component to it, and in particular a heavy component, lies in the tendency of the plaster to crumble and the tendency of the layers the board to tear easily. This difficulty is overcome by a dowel of the type defined above provided, in addition, with a piercing end, either on the body or on the rocker, and a thread on the body for first of all fixing the dowel in the sheet and then preventing, after the fixing of the component, the rotation of the dowel in the event of the unscrewing of the fixing screw. Such a dowel could be produced by moulding the body in zinc and stamping the rocker from a piece of metal or again by moulding the body and rocker separately from plastic material.